Elven Thinblade (5e Equipment)
From D&D Wiki
Martial Melee Weapons
The elven thinblade has the fine blade and needle-like point of a rapier, but the length and strength of a longsword. Its thin, flexible blade slips easily into the seams of armor, or between the ribs of an enemy.
You are proficient with elven thinblades if you have the Elven Weapon Training racial feature, or the Elven Blade Master feat.
Special. You score a critical hit on a roll of 19-20 on the d20 with this sword.